Author Topic: L7807CV caps  (Read 4186 times)

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Offline Flump

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L7807CV caps
« on: July 10, 2014, 12:48:51 am »
In the datasheet for this regulator
it has an application circuit and lists
0.33uF and 0.1uF caps, are these values critical ?

i have the 0.1 but not the 0.33,
I do however have a 0.496uF
can I use that instead ?
 

Offline Simon

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Re: L7807CV caps
« Reply #1 on: July 10, 2014, 12:51:09 am »
More is ok less it not so you will be fine. It's usually best to use something like ceramics as they have best ESR but many regulators now state being able to work with electrolytics
https://www.simonselectronics.co.uk/shop New stock now in of EEVblog Brymen 235 and uCurrent Gold, Now selling a selection of Probe Master probes
 

Offline Flump

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Re: L7807CV caps
« Reply #2 on: July 10, 2014, 01:14:58 am »
thanks very much simon  :-+
 

Offline rob77

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Re: L7807CV caps
« Reply #3 on: July 10, 2014, 01:17:16 am »
i have a habit of putting 100nF ceramic to both input and output for all 78xx regulators - without checking the datasheet of the particular manufacturer ;) and actually never ran into issues (oscillations...etc...).. so relax and use the caps you have ;)
 

Offline Flump

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Re: L7807CV caps
« Reply #4 on: July 10, 2014, 02:42:26 am »
thanks rob  :)

I have run into a bit of a problem though
I made me thinggy up but its getting too hot to touch
and it doesnt have any heatshrink / box around it yet

its to run a tomtom on a motorbike, so 12/14 Vin
to 5 Vout

Is the heat because of Vin being much higher than Vout ?

(tomtom draws 3-400mA)





 

Offline Simon

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Re: L7807CV caps
« Reply #5 on: July 10, 2014, 02:58:22 am »
I think you need a bigger heatsink
https://www.simonselectronics.co.uk/shop New stock now in of EEVblog Brymen 235 and uCurrent Gold, Now selling a selection of Probe Master probes
 

Online Monkeh

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Re: L7807CV caps
« Reply #6 on: July 10, 2014, 03:30:29 am »
I think you need to use something more suitable than a linear regulator.
 

Offline Simon

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Re: L7807CV caps
« Reply #7 on: July 10, 2014, 03:31:42 am »
make a SMPS or even get a car one, if you have nothing to plug it into take it apart and wire it up.
https://www.simonselectronics.co.uk/shop New stock now in of EEVblog Brymen 235 and uCurrent Gold, Now selling a selection of Probe Master probes
 

Offline rob77

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Re: L7807CV caps
« Reply #8 on: July 10, 2014, 03:34:33 am »
 ah... it's for a motorbike....
 then you need some input filter/protection.... motorbikes/automobiles are a very harsh environment for electronics. it would be better to look after a car USB charger and use the guts of that charger. - definitely cheaper than building a regulator suitable for car/motorbike.
 

Offline Simon

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Re: L7807CV caps
« Reply #9 on: July 10, 2014, 03:38:52 am »
Definitely more efficient too, your wasting 60% or more of the power with a linear
https://www.simonselectronics.co.uk/shop New stock now in of EEVblog Brymen 235 and uCurrent Gold, Now selling a selection of Probe Master probes
 

Offline Flump

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Re: L7807CV caps
« Reply #10 on: July 10, 2014, 03:49:59 am »
I thought the car ones just used similar regulars
what or how do they do different ?
 

Online mariush

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Re: L7807CV caps
« Reply #11 on: July 10, 2014, 03:53:16 am »
The extra voltage is dissipated by the regulator as heat, so with 12v input and 5v output and 400mA, you're wasting (12-5) * 0.4 = 2.8w

The heatsink is too small for that much dissipated power.

Here's how to figure out how good of a heatsink you need. 

Go in datasheet and look up the thermal data, for example in this datasheet:  http://www.farnell.com/datasheets/1696820.pdf  you have it on page 7.

For TO-220 you have:

RthJC Thermal resistance junction-case  5°C/W
RthJA Thermal resistance junction-ambient : 50°C/W

You also have above the maximum operating range (TOP Operating junction temperature range )  which peaks up to 125-150c ... let's go with 125°C.
You have to determine how much can the regulator warm up from the default temperature when it's not working. Normally, you would say the default temperature as 25°C but since your regulator may be in the sun when you parked the motorbike and so on, it's safer to go with 40°C.
So you have now 125°C - 40°C to work with, or about 85°C.

With 12v input and 5v output and 400mA (0.4A), you're going to dissipate (12-5)* 0.4 = 2.8w  ... 

The regulator has a thermal resistance junction ambient of 50°C / watt meaning without anything connected to the metal back, for every watt of power the regulator will raise by 50°C.  In your case, 2.8w will increase the temperature by 2.8 x 50°C which is more than the 85°C margin you have, so it's obvious you need a heatsink.

or in another way.. 85°C / 2.8w = 30.35°C/w .. this is the maximul thermal resistance you have to reach by placing a heatsink on the regulator.

So now the heatsink specs must be :  30.35°C/w - resistance junction-case of 5°C/w - resistance of any thermal paste or silicon pad you put between regulator and heatsink (about 0.5°C/w)  - about 0.1°C/w resistance of the thermal paste itself.

Therefore, your heatsink must be rated at 30.35°C/w - 5°C/w - 0.6°C/w = ~ 24.75°C/w
Now that's the maximum value recommended, in practice your heatsink should be bigger... I don't think you want something running 24/7 at 100-110c on your motorbike.

You can go on Digikey or Farnell and sort heatsinks for TO-220 based on this number, 24.75°C/w or smaller.

As a side note: the math above is for heatsinks that are only helped by the air moving around in a case... warm air. If there's a fan blowing over the heatsink, it will work better. Since you mean a motorbike, if you're moving with the motorbike and air blows over the heatsink (you expose the heatsink to air) then heatink will run better. But on the other side, the heatsink may warm up from the bike engine or from sun shining on it all the time (i already counted on this a bit when I said 40c ambient temperature)

The heatsink you have looks like this one, which is rated for 17.4C : http://www.digikey.com/product-detail/en/581002B02500G/HS387-ND/1216391 
Your heatsink is probably at around 20c/w and as you can see, it keeps the regulator working but hot (lower than 125c).
To keep everything happy, you'd probably have to go with a heatsink with 10c/w or less, for example something like this: http://www.digikey.com/product-detail/en/RA-T2X-51E/RA-T2X-51E-ND/2416489

But in your case, it's better to just go with a switching regulator .. get a usb car charger and pop open the case and you have 9-15v+ input, 5v output with much higher efficiency ..
I made a video here about one I bought at sale for about 2$  (or you can make one yourself with the classic mc34063, it's not that hard) :


 


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